Summary of Significant Accounting Policies - Additional Information (Detail) - ASU 2016-09 [Member] - Federal and State Tax Authority [Member] $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Net operating loss deferred tax asset increased amount due to excess tax benefits	$ 14.9
Deferred tax asset valuation allowance increased amount	$ 14.9